Warming oceans: fish on the move

The oceans are getting warmer, and fish are adapting to rising ocean temperatures with their fins and swimming to waters that better suit their temperature preferences. Shifts in the distribution of important coastal fish species are resulting in changes to historical fishing options, new fishing opportunities and new fisheries management challenges.

There are many examples of the distributions of fish species changing in response to the warming oceans, but let’s focus on a few species important to Delaware both past and present. Winter flounder were popular recreational fish decades ago, but they have largely left Delaware waters that are now too warm. Summer flounder and black sea bass remain two of the most sought-after recreational fish in Delaware, but their distribution is gradually shifting northward, with fewer fish available in Delaware waters.

While flounder and black sea bass are examples of changing fish distributions that have decreased fishing opportunities in Delaware, other species seem to be more available in Delaware due to the warming ocean, such as cobia and blueline tilefish, with other southern species likely to become more common in Delaware as our waters continue to warm.

Changing temperatures, changing fish

Winter flounder were a popular, cool weather catch for Delaware’s recreational anglers during the 1960s and 1970s. Delaware was at the southern range of winter flounder distribution and their departure from Delaware waters began soon after those peak years. The number of winter flounder caught during December in Delaware’s Inland Bays by the Division of Fish and Wildlife’s trawl survey declined by more than 90 percent between 1966 and 1981.

Adult winter flounder had virtually disappeared from the trawl survey catches by the early 1990s, with a similar and notable reduction in recreational angler catches. Winter flounder have suffered a steep population decline over the past 20 years and the current winter flounder distribution suggests that this species is still moving north to stay in its preferred temperature range, making it unlikely that they will return to Delaware.

Summer flounder are found from Nova Scotia to Florida, but they are most abundant in the waters between Cape Cod to Cape Fear, N.C. While summer flounder abundance is still highest between Cape Cod and Cape Fear, in recent years a pronounced contraction of the population has occurred in the southern edge of its distribution in North Carolina and Virginia, with a concurrent northern expansion into Georges Bank north of Massachusetts.

Black sea bass, unlike the flounder species, has a thriving population that is growing as its range expands. However, like the flounder species, its distribution in Delaware waters is shrinking as inshore areas that it inhabited as recently as a decade ago have become too warm. Black sea bass are still highly sought by recreational anglers in Delaware, but black sea bass and summer flounder anglers alike now have to head out to deeper, cooler ocean waters to consistently catch these species. Delaware’s black sea bass recreational harvest is now occurring almost entirely from federal waters, which are ocean waters at least three miles offshore, instead of from state waters.

In addition to moving offshore to find preferred temperatures, black sea bass are definitely moving north. Most of the recreational black sea bass catch is now coming from New York and southern New England states rather than the Mid-Atlantic states. The black sea bass population in the Gulf of Maine has also exploded as those waters are now warm enough to support this species.

Warming waters will also result in southern fish species expanding their distributions north into Delaware. While southern and even sub-tropical fish species periodically make an appearance in Delaware waters during late summer, some of these species seem to be establishing themselves in Delaware and others are probably on their way.

Cobia are a highly prized southern game fish that were once rarely found north of Virginia. The center of distribution for cobia has moved northward from the Carolinas to the northern North Carolina and southern Virginia area. Delaware anglers reported catching so many cobia in 2016 that the Division of Fish and Wildlife added cobia to the Delaware Sportfishing Tournament for 2017.

Blueline tilefish are a deep-water fish that were not rare in the federal waters off of Delaware, but their population has grown to the point that Delaware, along with neighboring Mid-Atlantic states, established regulations to prevent overfishing in 2016. The Division of Fish and Wildlife added blueline tilefish to the Delaware Sportfishing Tournament in 2015 and saw the state record broken three times in that first year, an indication of its growing availability and popularity among Delaware anglers.

Management issues

These northern shifts in fish populations have presented fisheries management challenges. Coastwide or regional Fisheries Management Plans (FMPs) are used to manage all of these species, but these FMPs have not always kept up with the changing distribution of these species. Take summer flounder and black sea bass as examples.

The summer flounder population is jointly managed by the Atlantic Coast states through the Atlantic States Marine Fisheries Commission (ASMFC) as a single, coastwide population. For the recreational fishery, the summer flounder FMP gives each state from Massachusetts to North Carolina a portion of the summer flounder coast-wide quota based on their relative proportional recreational landings in 1998 when New Jersey was the center of summer flounder distribution. Now that the center of flounder distribution has moved north, the previous harvest quota allocation ratios seem to be over-allocating flounder quota to states with decreasing flounder abundance and under-allocating flounder quota to states with increasing flounder abundance. Since the coastwide summer flounder population has been declining for several years, all states must reduce their harvest, but those states that now have a larger proportion of the summer flounder population in their waters are forced to take larger relative reductions due to allocation ratios established almost 20 years ago. More recently, the FMP switched to regional rather than state-by-state allocation three years ago in an attempt to alleviate this harvest allocation problem.

The black sea bass population increase and northern range expansion has presented another management challenge as the Gulf of Maine states are not allowed to harvest black sea bass because they were not historically caught in the Gulf of Maine when the harvest quotas were established. There are fears in the Gulf of Maine that the increasing black sea bass population will adversely impact one of the most valuable fisheries in the nation, the Maine lobster fishery, because black sea bass are voracious predators of juvenile lobsters.

Temperature-related shifts in fish distribution means that some fish species that were long popular with Delaware anglers will become rarer here, but they may be replaced by other species that will present new opportunities for Delaware anglers. The shifting distribution of these fish species will also require fisheries management to show more foresight and be proactive in adapting to these new distributions as they are occurring.

John Clark is the Division of Fish and Wildlife Fisheries administrator.This article originally appeared in Outdoor Delaware magazine.

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